Acoustic interrogation of subsurface features tends to be limited by the frequency bandwidth of practical sources. High frequency signals have a relatively short penetration distance, while low frequency signals do not have collimation and generate unwanted signals within the well bore. It is difficult to generate a collimated acoustic beam signal in the sonic frequency range between about 15 kHz and about 120 kHz from the borehole to probe the rock formation surrounding a borehole with conventional transducers. Conventional sonic acoustic sources have large beam spread, such that as the frequency decreases, the beam spread increases. The beam spread also depends on the diameter of the transducer, which is limited by the borehole dimension. Sharp directivity steering for a particular frequency requires a number of conditions to be satisfied, including a long source array, uniform coupling of all the transducers to the rock formation around the borehole and knowledge of the acoustic velocities of the rock formation. In the borehole environment, these conditions are not often achievable because of underlying physics constraints, engineering feasibility or operating conditions, especially when the source signal has broad frequency bandwidth.
Traditional monopole and dipole borehole acoustic logs have been used to measure sonic velocity near the borehole using frequency range less than about 8 kHz. However, at this relatively low frequency, azimuthal resolution is relatively low. There are a number of patents that attempted to overcome this deficiency by using additional receivers to detect the direction of the signals returning to the receivers (see, for example, U.S. Pat. No. 5,544,127 and references cited within)). Applications for borehole sonic for reflection imaging, refraction imaging, fractures detection and permeability determination have also been proposed (see, for example, U.S. Pat. Nos. 5,081,611, 4,831,600, 4,817,059, and 4,797,859). All of these conventional techniques have operational and azimuthal resolution deficiency as the source lacks or has insufficient azimuthal directivity and desired frequency bandwidth.
For cement evaluation, ultrasonic waves in the frequency range of hundreds of kilohertz (e.g., low ultrasonic frequency range between 80 kHz and about 120 kHz and ultrasonic frequency range around about 200 kHz) have been used to detect a cement gap behind the casing. Even though frequencies around 200 kHz allow for good azimuth resolution, the distance range for detection at around this frequency is very limited, i.e., the depth of penetration to investigate behind the formation and channels between cement and rock formation is limited for ultrasonic source at frequency around 200 kHz. Conventional cement evaluation logs use a frequency of 30 kHz and can investigate deeper. However, these conventional cement evaluation logs lack azimuthal resolution because the wavelength is around the borehole radius and, consequently, the borehole modes would excite the entire borehole. As a result it is difficult to extract detailed azimuthal information of the cement bonding. In order to overcome this deficiency, multiple sources (emitting in the frequency range between 70 kHz and 120 kHz) and multiple receivers are used in a Sector Bond Tool (SBT) system. However, even with the use of multiple sources and multiple receivers, the conventional SBT system was not able to cure the deficiencies of the prior conventional cement evaluation logs as the source still lacked azimuthal directivity to effectively detect the existence of small channels between the cement and the rock formation.